Shift Registers

ABSTRACT

A shift register including shift register units controlled by first and second clock signals for generating an output signal. For each unit, in an active period, the first driving device drives the first switch device to activate the output signal, and the second driving device provides a voltage signal according to the first clock signal to drive the first switch device to de-activate the output signal. When the first switch device de-activates the output signal, the second switch device provides the voltage signal to serve as the output signal according to the second clock signal. In the active period, the voltage signal has a low level, and the first and second clock signals are set as alternating-current signals and are opposite to each other. In a blanking period, the voltage signal has a high level, and each of the first and second clock signals is set as a direct-current signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. application Ser. No.12/402,719, filed Mar. 12, 2009, and entitled “Shift Registers,” whichclaims the benefit of Taiwan application Ser. No. 97110961 filed Mar.27, 2008, the subject matters of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a shift register, and more particularly to acontrol method for compensating for shifting of threshold voltage oftransistors in the shift register.

2. Description of the Related Art

In current liquid crystal display panels, gate drivers and drain driversare arranged to provide scan signals and data signals. In order todecrease costs, a shift register which has the same function as a gatedriver is arranged in a glass panel. Most shift registers are formed byamorphous silicon thin-film processes. When a display panel is lit,transistors of a shift register in the display panel are affected bystress, and the display panel thus operates irregularly.

FIG. 1 shows a conventional shift register unit of a shift register.FIG. 2 is a timing chart of signals of the shift register unit inFIG. 1. Referring to FIGS. 1 and 2, a shift register unit 1 iscontrolled by clock signals CK and XCK opposite to each other, that is,the clock signals CK and XCK have inverse phases, and are coupled to alow voltage source Vss. The shift register unit 1 receives outputsignals S_(N−1) and S_(N+1) respectively from the previous shiftregister unit and the next shift register unit and generates an outputsignal S_(N). At a time point P10, the output signal S_(N−1) isactivated, that is, the output signal S_(N−1) is at a high level, and atransistor T10 is turned on. A voltage V_(N10) at a node N10 is changedto a high level according to the output signal S_(N−1) to turn ontransistors T11 and T12. At this time, since the clock signal CK is at alow level and the transistor T12 is turned on, a voltage V_(N11) at anode N11 is at a low level to turn off a transistor T13. A transistorT15 is turned on by the clock signal XCK with a high level, and theoutput signal S_(N) is de-activated, that is, the output signal S_(N) isat a low level.

At a time point P11, the output signal S_(N−1) is de-activated, and thetransistor T10 is turned off. The clock signal CK is changed to a highlevel. In the period between the time points P11 and P12, the clocksignal CK with the high level couples to the node N10 through acapacitor C10 and the transistor T13, so that the voltage V_(N10) at thenode N10 is raised to a higher level according to the clock signal CK toturn on the transistors T11 and T12. According to the low voltage sourceVss and the turned-on transistor T12, the voltage V_(N11) at the nodeN11 remains at the low level to turn off the transistor T13. The clocksignal CK with the high level is transmitted to an output node N12through the turned-on transistor T11 to serve as the output signalS_(N), in other words, the output signal S_(N) is activated. The clocksignal XCK with a low level turns off a transistor T15, and the voltageV_(N11) with the low level turns off a transistor T16. Accordingly, theoutput signal S_(N) can stably remain in the activated state.

At a time point P12, the clock signal CK is changed to a low level, andthe output signal S_(N+1) is activated to turn on the transistor T14.The voltage V_(N10) at the node N10 is gradually decreased according tothe low voltage source Vss to turn off the transistors T11 and T12. Atthis time, the clock signal XCK with a high level turns on thetransistor T15, so that the voltage of the low voltage source Vss isprovided to the output node N12 to serve as the output signal S_(N), inother words, the output signal S_(N) is de-activated.

At a time point P13, the clock signal CK is changed to a high level, andthe voltage V_(N11) at the node N11 is changed to a high level to turnon the transistor T13. Thus, the voltage N10 remains at a low level.Moreover, the voltage V_(N11) with the high level turns on thetransistor T16, so that the output signal S_(N) remains in thede-activated state. After the time point P13, the shift register unit 1operates according to the clock signal CK and XCK. The voltage V_(N10)at the node N11 is switched between a high level and a low level.

It is assumed that the high level and the low level of the clock signalCK is 15V and −9V respectively, and the voltage of the low voltagesource Vss is −7V. When the clock signal CK is at the high level to turnon the transistor T13, the voltage difference between a gate and asource of the transistor T13 is 22V. If the gate-source voltage Vgs ofthe transistor T13 is under positive base stress for a long time, thethreshold voltage of the transistor T13 shifts, and the voltages V_(N10)and V_(N11) become irregular, as shown by the dotted line in V_(N10) andV_(N11) in FIG. 2. Similarly, if the gate-source voltages Vgs of thetransistors T11, T12, and T14-16 are under positive base stress for along time, their threshold voltages also shift. Thus, when the thresholdvoltages of the transistors in the shift register unit 1 shift, theshift register unit 1 operates irregularly and outputs an incorrectoutput signal S_(N).

BRIEF SUMMARY OF THE INVENTION

An exemplary embodiment of a shift register operating in an activeperiod and a blanking period is provided, comprising a plurality ofsubstantially cascaded shift register units. Each of the shift registerunits is controlled by a first clock signal and a second clock signalfor generating an output signal. The output signal is periodicallyactivated. Each of the shift register units comprises first and secondswitch devices and first and second driving devices.

The first switch device provides the output signal through an outputnode. The first driving device drives the first switch device accordingto a first input signal to activate the output signal in the activeperiod. The second driving device is coupled to a voltage signal andprovides the voltage signal according to the first clock signal to drivethe first switch device to de-activate the output signal in the activeperiod. The second switch device is coupled to the voltage signal. Whenthe first switch device de-activates the output signal, the secondswitch device provides the voltage signal to the output node accordingto the second clock signal. In the active period, the voltage signal isat a low level, and the first and second clock signals are set asalternating-current signals and are opposite to each other. In theblanking period, the voltage signal is at a high level, and each of thefirst and second clock signals is set as a direct-current signal.

Another exemplary embodiment of a shift register comprises substantiallycascaded first, second, and third shift register units. Each of thefirst, second, and third shift register unit is controlled by a firstclock signal and a second clock signal opposite to each other forgenerating an output signal. The output signal is periodicallyactivated. Each of the first, second, and third shift register unitscomprises first and second switch devices and first and second devices.

The first switch device provides the output signal through an outputnode. The first driving device drives the first switch device accordingto a first input signal to activate the output signal. The seconddriving device is coupled to the second clock signal and provides thesecond clock signal according to the first clock signal to drive thefirst switch device to de-activate the output signal. The second switchdevice is coupled to the voltage signal. When the first switch devicede-activates the output signal, the second switch device provides thevoltage signal to the output node according to the second clock signal.

Another exemplary embodiment of a control method for a shift register isprovided. The shift register operates in an active period and a blankingperiod and comprises a plurality of substantially cascaded shiftregister units. Each of the shift register units is controlled by afirst clock signal and a second clock signal for generating an outputsignal. The output signal is periodically activated. Each of the shiftregister units comprises first and second switch devices and first andsecond driving devices. The first switch device provides the outputsignal through an output node. The second driving device and the secondswitch devices are coupled to a voltage signal. In the active period,the control method comprises: switching the voltage signal to a lowlevel and setting the first and second clock signals asalternating-current signals, wherein the first and second are oppositeto each other; driving the first switch device to activate the outputsignal by the first driving device according to a first input signal;providing the voltage signal to drive the first switch device tode-activate the output signal by the second driving device according tothe first clock signal; and providing the voltage signal to the outputnode by the second switch device according to the second clock signalwhen the first switch device de-activates the output signal. In theblanking period, the control method comprises switching the voltage to ahigh level, and setting each of the first and second clock signals as adirect-current signal.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 shows a conventional shift register unit of a shift register;

FIG. 2 is a timing chart of signals of the shift register unit in FIG.1;

FIG. 3 shows an exemplary embodiment of a shift register;

FIG. 4 shows an exemplary embodiment of a shift register unit;

FIG. 5 is a timing chart of signals of the shift register unit in FIG.4;

FIG. 6 is a flow chart of an exemplary embodiment of a control methodfor a shift register; and

FIG. 7 shows another exemplary embodiment of a shift register unit.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

Shift registers are provided. In an exemplary embodiment of a shiftregister in FIG. 3, a shift register 3 is applied in a liquid crystaldisplay panel and operates an active period and a blanking period.Referring to FIG. 3, the shift register 3 comprises a plurality ofsubstantially cascaded shift register units 30 ₁-30 _(M). Each of theshift register units 30 ₁-30 _(M) is controlled by clock signals CK andXCK and coupled to a voltage source. Each of the shift register units 30₁-30 _(M) receives a first input signal and a second input signal andgenerates an output signal according to the clock signals CK and XCK.Output signals S₁-S_(M) generated by the shift register units 30 ₁-30_(M) are substantially activated, and each of the output signalsS_(i)-S_(M) is periodically activated.

Each (30 _(N)) of the shift register units 30 ₁-30 _(M) receives anoutput signal S_(N−1) generated by the previous shift register units 30_(N−1) to serve as the first input signal and an output signal S_(N+1)generated by the next shift register units 30 _(N+1) to serve as thesecond input signal, wherein 1<N<M, and N is an integer. The outputsignals S_(N−1), S_(N), and S_(N+1) are substantially activated. Forexample, the shift register units 30 ₂ receives the output signal S₁generated by the previous shift register units 30 ₁ and the outputsignal S₃ generated by the next shift register units 30 ₃ and generatesthe output signal S₂. The output signal S₂ generated by the shiftregister units 30 ₂ is received by the next shift register units 30 ₃.

The shift register units 30 ₁, which is the first stage of the shiftregister 3, receives the output signal S₂ from the shift register units30 ₂ to serve as the second input signal. The shift register units 30 ₁further receives a driving signal S_(D) generated by an external orinternal circuit to serve as the first input signal. The driving signalS_(D), the output signal S₁, and the output signal S₂ are substantiallyactivated. Similarly, the shift register units 30 _(M), which is thelast stage of the shift register 3, receives the output signal S_(M−1)from the shift register units 30 _(M−1) to serve as the first inputsignal. The shift register units 30 _(M−1) further receives a controlsignal S_(C) generated by an external or internal circuit to serve asthe second input signal. The output signal S_(M−1), the output signalS_(M), and the control signal S_(C) are substantially activated.

FIG. 4 shows an exemplary embodiment of a shift register unit. In theembodiment in FIG. 4, the shift register unit 30 ₂ of the shift register3 is given as an example for description, and the other shift registerunits 30 ₁ and 30 ₃-30 _(M) have the same circuitry as the shiftregister units 30 ₂. The shift register units 30 ₂ receives the outputsignal S₁ generated by the previous shift register units 30 ₁ to serveas the first input signal and the output signal S₃ generated by the nextshift register units 30 ₃ serve as the second input signal.

The shift register unit 30 ₂ comprises driving devices 40-42, switchdevices 43-47, and a capacitor C40. In the embodiment, the drivingdevices 40-42 and the switch devices 43-47 are implemented respectivelyby NMOS transistors T40-T42 and T43-T47. Sources of the transistors T42and T44-T47 are coupled to a voltage source Vss. In the followingdescription, a signal state at a high level indicates that the signal isactivated, while a signal state at a low level indicates that the signalis de-activated. FIG. 5 is a timing chart of signals of the shiftregister unit in FIG. 4. The shift register unit 30 ₂ operates an activeperiod PA and a blanking period PB. In the active period PA, the voltagesource Vss provides a low-level voltage signal, and the clock signals CKand XCK are alternating-current signals and are opposite to each other,that is, the clock signals CK and XCK have inverse phases. In theblanking period PB, the voltage source Vss is changed to provide ahigh-level voltage signal, and the clock signals CK and XCK are changedto direct-current signals with a low level. The detailed operation ofthe shift register unit 30 ₂ is described in following.

At a time point P50 in the active period PA, the output signal S₁ ischanged to a high level, and a transistor T40 is turned on. A voltageV_(N40) at a node N40 is changed to a high level according to the outputsignal S₁ to turn on transistors T43 and T45. At this time, since theclock signal CK is at a low level and the transistor T45 is turned on, avoltage V_(N41) at a node N41 is at the low level to turn off atransistor T41. A transistor T46 is turned on by the clock signal XCKwith a high level, so that the output signal S₂ is at a low level, thatis, the output signal S₂ is de-activated.

At a time point P51 in the active period PA, the output signal S₁ ischanged to a low level, and the transistor T40 is turned off. The clocksignal CK is changed to a high level. Between the time points P51 andP52, the clock signal CK with the high level couples to the node N40through a capacitor C40 and the transistor T41, so that the voltageV_(N40) at the node N40 is raised to a higher level according to theclock signal CK to turn on the transistors T43 and T45. A low-levelvoltage signal provided by the voltage source Vss is transmitted to thenode N41 to turn off the transistor T41, that is, the transistor T41 isdisabled. The clock signal CK with the high level is transmitted to anoutput node N42 through the turned-on transistor T43 to serve as theoutput signal S₂, in other words, the output signal S₁ is activated bythe transistor T43. The low-level voltage signal provided by the voltagesource Vss is transmitted to the node N41, and voltage V_(N41) remainsat the low level to turn off the transistor T47. The clock signal XCKwith a low level turns off the transistor T46. Accordingly, the outputsignal S₁ can stably remain in the activated state.

At a time point P52 in the active period PA, the clock signal CK ischanged to the low level, and the output signal S₃ is activated to turnon the transistor T42. The voltage V_(N40) at the node N40 is graduallydecreased according to the low-level voltage signal of the voltagesource Vss to turn off the transistors T43 and T45, so that thetransistor T43 does not activate the output signal S₂. At this time, theclock signal XCK with the high level turns on the transistor T46, sothat the low-level voltage signal of the voltage source Vss is providedto the output node N42 to serve as the output signal S₂, in other words,the output signal S₂ is de-activated.

At a time point P53 in the active period PA, the clock signal CK ischanged to the high level, and the voltage V_(N41) at the node N41 ischanged to a high level to turn on the transistor T41. The low-levelvoltage signal of the voltage source Vss is coupled to the node N40through the turned-on transistor T41. Thus, the voltage V_(N40) at thenode N40 remains at a low level to turn off the transistor T43, so thatthe transistor T43 does not activate the output signal S₂. Moreover, thevoltage V_(N41) with the high level turns on the transistor T46, and thelow-level voltage signal of the voltage source Vss is provided to theoutput node N42 to serve as the output signal S₂. Thus, the outputsignal S₂ remains in the de-activated state. In the active period andafter the time point P53, the shift register unit 30 ₂ operatesaccording to the clock signal CK and XCK. The voltage V_(N41) at thenode N41 is switched between a high level and a low level.

It is assumed that the high level and the low level of the clock signalCK is 15V and −9V, respectively, and the voltage signal of the voltagesource Vss1 is −7V. In the active period PA, when the clock signal CK isat a high level to turn on the transistor T41, the voltage differencebetween a gate and a source of the transistor T41 is 22V, that is, thegate-source voltage Vgs of the transistor T41 is under large positivebase stress. Similarly, the gate-source voltages Vgs of the transistorsT42, T43, and T45-T47 are also under large positive base stress. Thelarge positive base stress results in the shifting of the thresholdvoltages of these transistors.

In the blanking period PB, the output signals S₁-S₃ are at a low level,and the clock signals CK and XCK are changed to direct-current signalswith a low level. Particularly, in the blanking period PB, the voltagesignal provided by the voltage source Vss is at the same level as thehigh level of the clock signal CK in the active period PA. Thus, thegate-source voltage Vgs of the transistor T41 is under a negative basestress in the blanking period PB, which induces compensation for theshifting of the threshold voltage of the transistor T41. Similarly, inthe blanking period PB the gate-source voltages Vgs of the transistorsT42, and T45-T47 are also under a negative base stress for compensationfor the shifting of the threshold voltage of the transistors.

In this embodiment, a gate and a source of the transistor T44 arecoupled to the voltage source Vss. In the blanking period PB, thehigh-level voltage signal of the voltage source Vss turns on thetransistor T44, so that the output node N42 is at a high level. Thus,the gate-source voltage Vgs of the transistor T43 is under a negativebase stress for compensation for the shifting of the threshold voltage.

FIG. 6 is a flow chart of an exemplary embodiment of a control methodfor a shift register. The control method is described according to FIGS.4-6. In an active period PA, the voltage signal provided by the voltagesource Vss is switched to a low level, and the clock signals CK and XCKare set as alternating-current signals (step S60). The driving device 40drives the switch device 42 to activate the output signal S₂ accordingto the output signal S₁ (step S61). The driving device 41 provides thevoltage signal of the voltage source Vss according to the clock signalCK to drive the switch device 43 to de-activate the output signal S₂(step s62). When the switch device 43 de-activates the output signal S₂,the switch device 46 provides the voltage signal of the voltage sourceVss to the output node N42 according to the clock signal XCK for servingas the output signal S₂ (step S63). In a blanking period PB, the voltagesignal of the voltage source Vss is switched to a high voltage level,and the clock signals CK and XCK are set as direct-current signals witha low level (step S64).

FIG. 7 shows another exemplary embodiment of a shift register unit. Inthe embodiment in FIG. 7, the shift register unit 30 ₂ of the shiftregister 3 is given as an example for description. In FIGS. 4 and 7, thesame elements and the same signals are represented by the same labels.Referring to FIGS. 4 and 7, a majority of the element connections andthe signal timings are the same. One difference between FIGS. 4 and 7 isthat the shift register of FIG. 7 does not comprise the switch device44. Another difference is that the sources of the transistors T41, T42,and T45-T47 of FIG. 4 are coupled to the voltage source Vss, while thesource of the transistor T41 of FIG. 7 is coupled to the clock signalXCK and not the voltage source Vss. Moreover, the voltage signalprovided by the voltage source Vss is at a low level and is not changedto a high level. When the transistor T41 is turned on according to theclock signal CK, the clock signal XCK is transmitted to the node N41 toturn off the transistor T43, so that the transistor T43 does notde-activate the output signals S₂.

The source of the transistor T41 is coupled to the clock signal XCK, andthe gate thereof is coupled to the clock signal CK. Thus, in the activeperiod, the gate-source voltage Vgs of the transistor T41 is alternatelyunder a positive base stress and a negative base stress, reducing theeffect from the shifting of the threshold voltage of the transistor T41.

According to the embodiments, the shifting of the threshold voltages oftransistors, which resulted from a positive base stress, can be reducedby a negative base stress on the transistors.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A shift register, comprising a plurality of substantially cascadedshift register units, each controlled by a first clock signal and asecond clock signal opposite to each other for generating an outputsignal, wherein the output signal is periodically activated, and each ofthe shift register units comprises: a first switch device for providingthe output signal through an output node; a first driving device fordriving the first switch device according to a first input signal toactivate the output signal; a second driving device, coupled to thesecond clock signal, for providing the second clock signal according tothe first clock signal to drive the first switch device to de-activatethe output signal; and a second switch device, coupled to the voltagesignal, for providing the voltage signal to the output node according tothe second clock signal when the first switch device de-activates theoutput signal.
 2. The shift register as claimed in claim 1, wherein eachof the shift register units further comprises a third driving device,coupled to the voltage signal, for driving the first switch device tode-activate the output signal by the voltage signal according to asecond input signal, and wherein each of the first and second inputsignals is periodically activated, and the first input signal, theoutput signal, and the second input signal are substantially activated.3. The shift register as claimed in claim 2, wherein the plurality ofshift register units comprises first, second, and third substantiallycascaded shift register units, the output signal of the first shiftregister unit serves as the first input signal of the second shiftregister unit, the output signal of the second shift register unitserves as the first input signal of the third shift register unit andthe second input signal of the first shift register, and the outputsignal of the third shift register unit serves as the second inputsignal of the second shift register unit.
 4. The shift register asclaimed in claim 1, wherein each of the shift register units furthercomprises a third switch device, coupled to the voltage signal, forproviding the voltage signal to the output node according to the firstclock signal when the first switch device de-activates the outputsignal.
 5. The shift register as claimed in claim 1, wherein each of theshift register units further comprises a fourth switch device, coupledto the voltage signal, for disabling the second driving device by thevoltage signal when the first driving device drives the first switchdevice to activate the output signal.
 6. The shift register as claimedin claim 1, wherein the plurality of shift register units comprisesfirst, second, and third substantially cascaded shift register units,the output signal of the first shift register unit serves as the firstinput signal of the second shift register unit, and the output signal ofthe second shift register unit.
 7. The shift register as claimed inclaim 1, wherein the voltage signal is at a low level.